1. Field of the Invention
The present invention relates to a photoelectric sensor circuit, which is conveniently used for a solar sensor disposed in an auto air conditioner system in a vehicle and for a light-receiving sensor disposed in an automatic light control system in the vehicle.
2. Description of Related Art
In general, a photodiode is used as a photoelectric sensing element in a conventional photoelectric sensor circuit. For instance, when the photodiode is used as a solar sensor disposed in an automatic air conditioner system in a vehicle, an amplifier circuit is not needed because the photodiode is used in a high illuminance range and outputs high level output currents (photoelectric currents). On the other hand, when the photodiode is used as a light-receiving sensor disposed in an automatic light control system in the vehicle, the amplifier circuit is needed because the photodiode is used in a low illuminance range and outputs low level output currents (photoelectric currents).
Both the solar sensor and the light-receiving sensor are usually disposed in a same area in the vehicle, for instance, on a dashboard, so it has been required to use the photodiode as both the solar sensor and the light-receiving sensor to decrease the number of parts in the photoelectric sensor circuit. In order to satisfy the above requirement, one terminal for directly outputting photoelectric currents from the photodiode and another terminal for outputting photoelectric currents amplified by the amplifier circuit are needed in the photoelectric sensor circuit. In this case, if the amplifier circuit is constructed as a current mirror circuit, the photoelectric sensor circuit has some advantages, for instance, it is easy to dispose both terminals in the photoelectric sensor circuit, and it is not necessary for the amplifier circuit to dispose a particular power source.
A conventional photoelectric sensor circuit which amplifies photoelectric currents from a photodiode by means of a current mirror circuit is disclosed in U.S. Pat. No. 5,004,901 (Yoshimoto et al.). As shown in FIG. 5 illustrated in Yoshimoto et al., a photodiode 1 receives reverse bias voltage from a power terminal +V.sub.cc. A current mirror circuit includes a transistor 3 in which most photoelectric currents from the photodiode 1 flow as collector currents I.sub.9 therein, five transistors 4-8 in which collector currents I.sub.10 -I.sub.14 having the same magnitude as the collector currents I.sub.9 flow therein, and a transistor 2 which amplifies part of the photoelectric currents from the photodiode 1 and provides the transistors 3-8 with them as base currents. According to these structures, the magnitudes of the output currents I.sub.out are five times as large as the photoelectric currents from the photodiode 1, so the current mirror circuit operates as an amplifier circuit, and a specialized power source is not necessary for the amplifier circuit.
The inventors of the present invention conceived of another photoelectric sensor circuit as illustrated in FIG. 6 (not known), based on the above photoelectric sensor circuit described in U.S. Pat. No. 5,004,901 so that the photoelectric sensor circuit directly outputs the photoelectric currents from the photodiode 1. As shown in FIG. 6, a photoelectric sensor circuit includes an output terminal 11 in which currents I.sub.out' which depend on photoelectric currents from the photodiode 1 flow therein. According to the photoelectric sensor circuit illustrated in FIG. 6, the photodiode 1 can be used as both a solar sensor and a light-receiving sensor and the photoelectric sensor circuit can be used as a unit that unifies both the solar sensor and the light-receiving sensor.
Considering circuits disposed in a vehicle, it is desirable to minimize the number of wires which electrically connect one circuit terminal to another circuit terminal. Of course, it is necessary to decrease the number of terminals in the circuit in order to decrease the number of wires. However, if the photoelectric sensor circuit is constructed to output two kinds of photoelectric currents, the photoelectric sensor circuit needs four terminals as shown in FIG. 6, that is, a power terminal +V.sub.cc, two output terminals 10, 11 and a ground terminal 13. Thus, it is necessary to decrease the number of wires in the photoelectric sensor circuit even if the photoelectric sensor circuit includes one terminal for directly outputting the photoelectric currents from the photodiode and another terminal for outputting the amplified the photoelectric currents.